The use of powdered metals, and particularly iron and its alloys, is known for forming magnets, such as soft magnetic cores for transformers, inductors, AC and DC motors, generators, and relays. An advantage to using powdered metals is that forming operations, such as compression or injection molding and/or sintering techniques, can be used to form intricate molded part configurations, such as magnetic cores, without the need to perform additional machining and piercing operations. As a result, the formed part is often substantially ready for use within its working environment as formed by the molding process.
Molded magnetic cores for AC applications generally should have low magnetic core losses. To provide low core losses, the individual metal particles within the magnetic core must be electrically insulated from each other. Numerous types of insulating materials, which also act as the binder required for molding, have been suggested by the prior art, including inorganic materials such as iron phosphate and alkali metal silicate, as well as an extensive list of organic polymeric materials. It is also known to coat a powdered metal with an inorganic undercoating and then provide an organic topcoat. In addition to providing adequate insulation and adhesion between the metal particles upon molding, the coating material should also have the ability to provide sufficient lubrication during the molding operation so as to enhance the flowability and compressibility of the particles, and therefore enable the particles to attain maximum density and strength.
As noted above, the individual metal particles of a magnetic core for AC applications must be electrically insulated from each other to provide low core losses. Because the insulating material must remain within the magnetic core, a magnetic core's maximum operating temperature will often be determined by the heat resistant properties of the material used to adhere the metal particles together. If the magnetic core is exposed to a temperature which exceeds the heat deflection temperature of the coating material, the ability of the coating material to encapsulate and adhere the particles will likely be degraded, which could ultimately destroy the magnetic core. Even where physical destruction of the magnetic core does not occur, the magnetic field characteristics of the magnetic core will likely be severely impaired because of the degradation of the insulating capability of the coating material due to the elevated temperatures.
As disclosed in U.S. patent application Ser. No. 07/710,427, now U.S. Pat. No. 5,211,896, filed Jun. 7, 1991, to Ward et al. and assigned to the assignee of the present invention, polyetherimide, polyethersulfone and polyamideimide have been found to perform well as the coating material for powdered iron and/or powdered iron alloys, so as to form insulated magnetic cores, particularly with respect to the ability to bind the iron particles together and resist thermal and chemical attack, and the ability to serve as a lubricant during the compression molding process. In addition, these polymers adhere well to the underlying metal particle. These polymers are soluble in a solvent to permit their application to the iron particles using a fluidized bed process which is known in the art.
However, a shortcoming associated with the teachings of Ward et al. is that polyetherimide and polyethersulfone have operating temperatures, as defined by their heat deflection temperatures, which may be insufficient for use in high temperature applications of greater than about 200.degree. C. As a result, magnetic cores formed with these materials may have a limited temperature capability. While polyamideimide has a higher heat deflection temperature of about 280.degree. C., and thus a higher operating temperature limit, developing optimum high temperature mechanical properties requires post-curing through an extended, closely controlled temperature cycle. Such a requirement tends to make polyamideimide impractical from a manufacturing and economic standpoint. Further, in some instances, these polymers may not compression mold suitably for certain applications due to insufficient lubricity. As a consequence, the magnetic cores may have unsuitably low densities, which corresponds to lower magnetic permeability.
As disclosed in U.S. patent application Ser. No. 07/915,587 filed Jul. 20, 1992, now U.S. Pat. No. 5,271,891 to Gay and assigned to the assignee of the present invention, polyphenylene oxide exhibits sufficient lubricity to significantly enhance the compressibility of metal particles coated with the polyphenylene oxide during molding and particularly during sintering operations. In addition, the polyphenylene oxide performs well as an insulating material and is particularly resistant to thermal and chemical attack. However, the polyphenylene oxide shares a common shortcoming with polyetherimide and polyethersulfone, in that the polyphenylene oxide generally has a useful temperature of no more than about 200.degree. C., thereby limiting its use at higher operating temperatures.
Thus, it would be desirable to provide a coating for powdered metals which has the ability to withstand relatively high operating temperatures, for molding magnetic cores from such coated metal particles, such that the mechanical properties and desired magnetic characteristics of the molded magnetic core do not deteriorate at high temperature applications. In addition, the coating should be soluble in a suitable solvent, and capable of improving lubrication during the molding process and providing adhesion between the metal particles after molding, so as to attain maximum density and strength of the as-molded article.